The invention relates to a biaxially oriented polyester film having a base layer B at least 80% by weight of which is composed of a thermoplastic polyester, and having two outer layers A and C. The invention also relates to the use of the film and to a process for its production.
In many cases, there is demand for food and drink packaging to have a high barrier effect with respect to gases, water vapor and flavors. For this reason, use is usually made of polypropylene films which have been metalized or have been coated with polyvinylidene chloride (PVDC). However, metalized polypropylene films are not transparent and are therefore not used in cases where the view of the contents is likely to have added promotional effect. Although films coated with PVDC are transparent, the coating, like the metalizing, takes place in a second operation which makes the packaging markedly more expensive. Ethylene-vinyl alcohol copolymers (EVOH) likewise exhibit a strong barrier effect. However, films modified with EVOH are particularly highly sensitive to moisture, and this limits their range of application. In addition, because of their poor mechanical properties they have relatively high thickness or have to be laminated with other materials at high cost, and they are also difficult to dispose of after use. Many packaging systems, furthermore, demand sealability of the films as well as an effective barrier.
It is therefore an object of the present invention to provide a transparent, sealable, biaxially oriented polyester film which has a high oxygen barrier, is simple and cost-effective to produce, has the good physical properties of the known films, and does not give rise to disposal problems.
The object is achieved by means of a biaxially oriented polyester film having at least three layers and having a base layer B at least 80% by weight of which is composed of (at least) one thermoplastic polyester, and having two outer layers A and C, wherein the outer layer A is composed of a polymer or a mixture of polymers which comprises at least 40% by weight of ethylene 2,6-naphthalate units and up to 40% by weight of ethylene terephthalate units and/or up to 60% by weight of units from aliphatic, including cycloaliphatic, or aromatic diols and/or dicarboxylic acids, and the opposite outer layer C has a sealing initialization temperature of xe2x89xa6200xc2x0 C., preferably xe2x89xa6130xc2x0 C., or between from about 100xc2x0 C. to 200xc2x0 C. The novel film generally has an oxygen permeability of less than 80 cm3/(m2 bar d), preferably less than 75 cm3/(m2 bar d), particularly preferably less than 70 cm3/(m2 bar d). The novel film is preferably transparent.
Preference is given to a polyester film in which the polymers of the outer layer A comprise at least 65% by weight of ethylene 2,6-naphthalate units and up to 35% by weight of ethylene terephthalate units. Among these, particular preference is in turn given to a polyester film of the type in which the polymers of the outer layer A comprise at least 70% by weight of ethylene 2,6-naphthalate units and up to 30% by weight of ethylene terephthalate units. The outer layer A may, however, also be composed completely of ethylene 2,6-naphthalate polymers.
Examples of suitable aliphatic diols are diethylene glycol, triethylene glycol, aliphatic glycols of the formula HOxe2x80x94(CH2)nxe2x80x94OH, where n is an integer from 3 to 6 (in particular 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol), or branched aliphatic glycols having up to 6 carbon atoms, and cycloaliphatic diols having one or more rings and if desired containing heteroatoms. Among the cycloaliphatic diols, mention may be made of cyclohexanediols (in particular 1,4-cyclohexanediol). Examples of suitable aromatic diols are those of the formula HOxe2x80x94C6H4xe2x80x94Xxe2x80x94C6H4xe2x80x94OH where X is xe2x80x94CH2xe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94C(CF3)2xe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94SO2xe2x80x94. Besides these, bisphenols of the formula HOxe2x80x94C6H4xe2x80x94C6H4xe2x80x94OH are also very suitable.
Preferred aromatic dicarboxylic acids are benzenedicarboxylic acids, naphthalenedicarboxylic acids (for example naphthalene-1,4- or -1,6-dicar-boxylic acid), biphenyl-x,xxe2x80x2-dicarboxylic acids (in particular biphenyl-4,4xe2x80x2-dicarboxylic acid), diphenylacetylene-x,xxe2x80x2-dicarboxylic acids (in particular diphenylacetylene4,4xe2x80x2-dicarboxylic acid) or stilbene-x,xxe2x80x2-dicarboxylic acids. Among the cycloaliphatic dicarboxylic acids, mention may be made of cyclohexanedicarboxylic acids (in particular cyclohexane-1,4-dicarboxylic acid). Among the aliphaticdicarboxylic acids, the C3-C19-alkanedioic acids are particularly suitable, where the alkane moiety may be straight-chain or branched.
The present invention also provides a process for producing this film. It encompasses
a) producing a film from base and outer layers A and C by coextrusion,
b) biaxial orientation of the film and
c) heat-setting of the oriented film.
To produce the outer layer A, it is expedient to feed granules of polyethylene terephthalate and polyethylene 2,6-naphthalate directly to the extruder in the desired mixing ratio. At about 300xc2x0 C. and with a residence time of about 5 min, the two materials can be melted and can be extruded. Under these conditions, transesterification reactions can occur in the extruder and during these copolymers are formed from the homopolymers.
The polymers for the base layer B are expediently fed in via another extruder. Any foreign bodies or contamination which may be present can be filtered off from the polymer melt before extrusion. The melts are then extruded through a coextrusion die to give flat melt films and are layered one upon the other. The coextruded film is then drawn off and solidified with the aid of a chill roll and other rolls if desired.
The biaxial orientation is generally carried out sequentially. For the sequential stretching, it is preferable to orient firstly in a longitudinal direction (i.e. in the machine direction) and then in a transverse direction (i.e. perpendicularly to the machine direction). This causes an orientation of the molecular chains. The orientation in a longitudinal direction may be carried out with the aid of two rolls running at different speeds corresponding to the stretching ratio to be achieved. For the transverse orientation, use is generally made of an appropriate tenter frame.
The temperature at which the orientation is carried out can vary over a relatively wide range and depends on the properties desired in the film. In general, the longitudinal stretching is carried out at from 80 to 130xc2x0 C., and the transverse stretching at from 90 to 150xc2x0 C. The longitudinal stretching ratio is generally in the range from 2.5:1 to 6:1, preferably from 3:1 to 5.5:1. The transverse stretching ratio is generally in the range from 3.0:1 to 5.0:1, preferably from 3.5:1 to 4.5:1.
During the subsequent heat-setting, the film is held for from 0.1 to 10 s at a temperature of from 150 to 250xc2x0 C. The film is then wound up in a conventional manner.
A great advantage of this process is that it is possible to feed the extruder with granules, which do not block the machine.
The base layer B of the film is preferably composed to an extent of at least 90% by weight of the thermoplastic polyester. Polyesters suitable for this are those made from ethylene glycol and terephthalic acid (=polyethylene terephthalate, PET), from ethylene glycol and naphthalene-2,6-dicarboxylic acid (=polyethylene 2,6-naphthalate, PEN), from 1,4-bishydroxymethylcyclo-hexane and terephthalic acid (=poly-1,4-cyclohexanedimethylene terephthalate, PCDT), and also from ethylene glycol, naphthalene-2,6-dicarboxylic acid and biphenyl4,4xe2x80x2-dicarboxylic acid (=polyethylene 2,6-naphthalate bibenzoate, PENBB). Particular preference is given to polyesters which are composed to an extent of at least 90 mol %, preferably at least 95 mol %, of ethylene glycol units and terephthalic acid units or of ethylene glycol units and naphthalene-2,6-dicarboxylic acid units. The remaining monomer units are derived from other diols and/or dicarboxylic acids. Examples of suitable diol comonomers are diethylene glycol, triethylene glycol, aliphatic glycols of the formula HOxe2x80x94(CH2)nxe2x80x94OH, where n is an integer from 3 to 6, branched aliphatic glycols having up to 6 carbon atoms, aromatic diols of the formula HOxe2x80x94C6H4xe2x80x94Xxe2x80x94C6H4xe2x80x94OH where X is xe2x80x94CH2xe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94C(CF3)2xe2x80x94, xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94or xe2x80x94SO2xe2x80x94,or bisphenols of the formula HOxe2x80x94C6H4xe2x80x94C6H4xe2x80x94OH.
The dicarboxylic acid comonomer units are preferably derived from benzenedicarboxylic acids, naphthalenedicarboxylic acids, biphenyl-x,xxe2x80x2-dicarboxylic acids (in particular biphenyl4,4xe2x80x2-dicarboxylic acid), cyclohexane-dicarboxylic acids (in particular cyclohexane-1,4-dicarboxylic acid), diphenylacetylene-x,xxe2x80x2-dicarboxylic acids (in particular diphenylacetylene-4,4xe2x80x2-dicarboxylic acid), stilbene-x,xxe2x80x2-dicarboxylic acid or C1-C16-alkane-dicarboxylic acids, where the alkane moiety may be straight-chain or branched.
The polyesters may be prepared by the transesterification process. The starting materials for this are dicarboxylic esters and diols, which are reacted using the customary transesterification catalysts, such as salts of zinc, of calcium, of lithium and of manganese. The intermediates are then polycondensed in the presence of widely used polycondensation catalysts, such as antimony trioxide or titanium salts. The preparation may be carried out just as successfully by the direct esterification process in the presence of polycondensation catalysts, starting directly from the dicarboxylic acids and thediols.
For processing the polymers, it has proven useful to select the polymers for the base layer and the outer layer(s) in such a way that the viscosities of the respective polymer melts do not differ excessively. Otherwise it is likely that there will be flow disturbances or streaks on the finished film. To describe the viscosity ranges of the two melts, use is made of a modified solution viscosity (SV). The solution viscosity is a measure of the molecular weight of the respective polymer and correlates with the melt viscosity. The chemical make-up of the polymer used may result in other correlations. For commercially available polyethylene terephthalates which are suitable for producing biaxially oriented films, the SVs are in the range from 600 to 1000. To ensure satisfactory film quality, the SV of the copolymers for the outer layer should be in the range from 300 to 900, preferably between 400 and 800, in particular between 500 and 700. If desired, a solid phase condensation may be carried out on the respective granules in order to adjust the SV of the materials as necessary. It is a general rule that the melt viscosities of the polymer melts for base and outer layer(s) should differ by not more than a factor of 5, preferably not more than a factor of from 2 to 3.
The polymers for the outer layer A may be prepared in three different ways:
a) In the copolycondensation, terephthalic acid and naphthalene-2,6-dicarboxylic acid are placed in a reactor together with ethylene glycol, and polycondensed to give a polyester, using the customary catalysts and stabilizers. The terephthalate and naphthalate units are then randomly distributed in the polyester.
b) Polyethylene terephthalate (PET) and polyethylene 2,6-naphthalate (PEN), in the desired ratio, are melted together and mixed, either in a reactor or preferably in a melt kneader (twin-screw kneader) or in an extruder. Immediately after the melting, transesterification reactions between the polyesters begin. Initially, block copolymers are obtained, but as reaction time increasesxe2x80x94depending on the temperature and mixing action of the agitatorxe2x80x94the blocks become smaller, and long reaction times give a random copolymer. However, it is not necessary and also not always advantageous to wait until a random distribution has been achieved, since the desired properties are also obtained with a block copolymer. The resultant copolymer is then extruded from a die and granulated.
c) PET and PEN are mixed as granules in the desired ratio, and the mixture is fed to the extruder for the outer layer. Here, the transesterification to give the copolymer takes place directly during the production of the film. This process has the advantage of being very cost-effective, and generally gives block copolymers, the block length being dependent on the extrusion temperature, the mixing action of the extruder and the residence time in the melt.
In a preferred embodiment of the invention, from 0.1 to 20% by weight of the polymers of the base layer are identical with those of the outer layer. These are either directly admixed with the base layer during extrusion or are in any case present in the film due to addition of regenerated material. The proportion of these copolymers in the base layer is selected in such a way that the base layer has a partially crystalline character.
The novel film exhibits a surprisingly high oxygen barrier. If, in contrast, the polymers used for the outer layer A comprise less than 40% by weight of ethylene 2,6-naphthalate units and more than 40% by weight of ethylene terephthalate units, then in some cases, although the film has somewhat lower oxygen transmission than a standard polyester film (composed to an extent of 100% by weight of polyethylene terephthalate), the transmission is still much too high. It has even been found that the oxygen barrier is poorer than in a standard polyester film if the outer layer comprises from 30 to 40% by weight of ethylene 2,6-naphthalate units and from 60 to 70% by weight of ethylene terephthalate units. However, even under these circumstances there may be advantage in a film having an outer layer A which comprises at least 5%, preferably between 5 and 40%, by weight of ethylene 2,6-naphthalate units and more than 40% by weight of ethylene terephthalate units, if the oxygen barrier does not play a decisive part in the application concerned.
It is expedient if, besides the base layer, the outer layer A is also partly crystalline. Surprisingly, it has been found that the crystallinity of the outer layers in particular influences the barrier properties of the film. If the crystallinity is high, the barrier is also high. The crystallinity of the film can be determined using FTIR spectroscopy, and for determining the crystallinity of the outer layer the measurements should be made in ATR mode. The crystallinity is shown by the presence of specific bands. The form of the crystallites (xcex1 form or xcex2 form) does not appear to be critical. For copolyesters containing ethylene units, the bands for the symmetrical CH-bond vibrations are at 2990 and 2971 cmxe2x88x921 (xcex1 form) and 2998 and 2971 cmxe2x88x921 (xcex2 form), respectively [see F. Kimura et al., Journal of Polymer Science: Polymer Physics, Vol. 35, pp. 2041-2047, 1997].
The base layer B and the outer layers A and C may, in addition, comprise customary additives, such as stabilizers and antiblocking agents. They are expediently added to the polymer or to the polymer mixture before melting takes place. Examples of stabilizers are phosphorus compounds, such as phosphoric acid and phosphoric esters. Typical antiblocking agents (also termed pigments in this context) are inorganic and/or organic particles, for example calcium carbonate, amorphous silica, talc, magnesium carbonate, barium carbonate, calcium sulfate, barium sulfate, lithium phosphate, calcium phosphate, magnesium phosphate, aluminum oxide, LiF, the calcium, barium, zinc and manganese salts of the dicarboxylic acids used, carbon black, titanium dioxide, kaolin, crosslinked polystyrene particles or crosslinked acrylate particles.
The additives selected may also be mixtures of two or more different antiblocking agents or mixtures of anti-blocking agents of the same make-up but of different particle size. The particles may be added to the individual layers in the customary concentrations, e.g. as glycolic dispersion during the polycondensation or via masterbatches -during extrusion. Pigment concentrations of from 0.0001 to 5% by weight have proven particularly suitable. A detailed description of the antiblocking agents is found, for example, in EP-A-0 602 964.
The film may be coated and/or corona- or flame-pretreated to establish other desired properties. Typical coatings are layers which promote adhesion, are antistatic, improve slip or have release action. These additional layers may be applied to the film by in-line coating using aqueous dispersions, before the transverse orientation.
The novel polyester film also comprises the second sealable outer layer C. The structure, thickness and make-up of the second outer layer C may be selected independently of the outer layer A already present, and the second outer layer C may likewise comprise the abovementioned polymers or polymer mixtures, but these are not substantially identical with those of the first outer layer. A precondition for achieving the sealability with the stated sealing initialization temperature of xe2x89xa6130xc2x0 C. is that the outer layer C, in contrast to the outer layer A, is amorphous.
When using the polymers given for the outer layer A, this is achieved by the presence of a proportion of from 25 to 75% by weight of ethylene 2,6-naphthalate units and a proportion of from 75 to 25% by weight of ethylene terephthalate units and/or of up to 50% by weight of units from cycloaliphatic or aromatic diols and/or dicarboxylic acids. In a preferred embodiment, the proportion of ethylene 2,6-naphthalate units is from 30 to 70% by weight, and in a particularly preferred embodiment it is from 35 to 65% by weight.
The hot-sealable layer may, however, also be composed of other polyester resins. Preference is given to copolyester resins which have been derived from aromatic dicarboxylic acids, such as terephthalic acid, isophthalic acid and hexahydroterephthalic acid, and from glycols, such as ethylene glycol, diethylene glycol, triethylene glycol or neopentyl glycol. Typical copolyesters which permit sealability with the sealing initialization temperature given are made of ethylene terephthalate and ethylene isophthalate, the proportion of ethylene terephthalate being from 50 to 90% by weight and the proportion of ethylene isophthalate being from 50 to 10% by weight. Preferred ranges are from 65 to 85% by weight of ethylene terephthalate and from 35 to 15% by weight of ethylene isophthalate, as described, for example, in EP-A-0 515 096 and EP-A-0 035 835.
The second outer layer C may also comprise other common outer layer polymers.
Between the base layer B and the outer layers A and C, there may also be an intermediate layer if desired. It may be composed of the polymers described for the base layers. In a particularly preferred embodiment, it is composed of the polyester used for the base layer. It may also comprise the customary additives described. The thickness of the intermediate layer is generally greater than 0.3 xcexcm and is preferably in the range from 0.5 to 15 xcexcm, in particular from 1.0 to 10 xcexcm.
The thickness of the outer layers A and C is generally greater than 0.1 xcexcm and is preferably in the range from 0.2 to 6.0 xcexcm, more preferably in the range from 0.3 to 5.5 xcexcm, in particular from 0.3 to 5.0 xcexcm. It is possible for the outer layers to have identical or different thicknesses.
The total thickness of the novel polyester film may vary within wide limits and depends on the application envisaged. It is preferably from 4 to 100 xcexcm, in particular from 5 to 50 xcexcm, preferably from 6 to 30 xcexcm, the base layer preferably presenting a proportion of from about 40 to 90% of the total thickness.
A further advantage is that the production costs of the novel film are only insignificantly greater than those of a film made from standard polyester raw materials. The other properties of the novel film which are relevant to processing and use remain essentially unchanged or are even improved. In addition, it has been ensured that regenerated material can be used in a proportion of up to 50% by weight, preferably from 10 to 50% by weight, based on the total weight of the film in each case, in the production of the film without significant adverse effect on its physical properties.
The film has excellent suitability for packaging foods and other consumable items.
For this intended use, the films are usually metalized or ceramically (e.g. SiOx, AlxOy, Na2SiO4, etc.) coated. Surprisingly, it has been found that the barrier is significantly better than in conventional metalized or ceramically coated polyester films if the metal layer or ceramic layer is applied to the PEN-containing outer layer A of the novel film. Conventional metalized polyester films have barrier values of  greater than 0.6 cm3/m2xc2x7dxc2x7bar, whereas the metalized novel films have barrier values of  less than 0.1 cm3/m2xc2x7dxc2x7bar. It has generally been found that, whatever the type of coating, the barrier effect of the novel films is better by a factor of 10 than that of conventional polyester films.
It has been found here that even a low outer layer thickness of  less than 1.5 xcexcm, preferably  less than 1.0 xcexcm, is sufficient to create the good barrier mentioned.
Other application sectors for the novel films are can liners, lid films (e.g. lids for yogurt cups, etc.) and thermo-transfer ribbons.
The following methods were used to characterize the raw materials and the films:
The oxygen barrier was measured using a Mocon Modern Controls (USA) OX-TRAN 2/20 in accordance with DIN 53 380, Part 3.
The SV (solution viscosity) was determined by dissolving a specimen of polyester in a solvent (dichloroacetic acid, 1% strength by weight solution). The viscosity of this solution and that of the pure solvent were measured in an Ubbelohde viscometer. The quotient (relative viscosity xcex7rel) was determined from the two values, 1.000 was subtracted from this, and the value multiplied by 1000. The result was the SV.
The coefficient of friction was determined according to DIN 53 375, 14 days after production.
The surface tension was determined using the xe2x80x9cink methodxe2x80x9d (DIN 53 364).
The haze of the film was measured in accordance with ASTM-D 1003-52. The Hxc3x6lz haze was determined by a method based on ASTM-D 1003-52, but in order to utilize the most effective measurement range, measurements were made on four pieces of film laid one on top of the other, and a 1xc2x0 slit diaphragm was used instead of a 4xc2x0 pinhole.
Gloss was determined in accordance with DIN 67 530. The reflectance was measured as an optical characteristic value for a film surface. Based on the standards ASTM-D 523-78 and ISO 2813, the angle of incidence was set at 20xc2x0 or 60xc2x0. A beam of light hits the flat test surface at the set angle of incidence and is reflected and/or scattered thereby. A proportional electrical variable is displayed representing light rays hitting the photoelectronic detector. The value measured is dimensionless and must be stated together with the angle of incidence.
Determination of the sealing initialization temperature
Hot-sealed specimens (seal seam 20 mmxc3x97100 mm) were produced with the Brugger HSG/ET sealing apparatus, by sealing a film at different temperatures with the aid of two heated sealing jaws at a sealing pressure of 10 N/cm2 and a sealing time of 0.5 s. From the sealed specimens, test strips of 15 mm width were cut. The T-seal seam strength was measured as in the determination of seal seam strength. The sealing initialization temperature is the temperature at which a seal seam strength of at least 0.5 N/15 mm is achieved.